Nike and the NFL continue to fight knockoff apparel on websites and outside stadiums.

Imitation may be the sincerest form of flattery, but there's big money at stake in this game. Illegal imitations plague all major U.S. professional sports leagues, to the tune of more than $13 billion in 2012, according to The Licensing Letter. And the NFL, with about $3 billion of that total, makes for an inviting target.

The NFL doesn't know how much is at stake with merchandise counterfeiters, a league attorney told a federal judge earlier this year. But the sale of counterfeit products "is very damaging to the NFL and its Member Clubs' reputation and goodwill," Anastasia Danias, NFL vice president of legal affairs wrote in a court brief, "as well as to the NFL's rightful market position."